The present invention relates to an arithmetic unit capable of encryption suitable for a contact usage and a non-contact usage incorporated into a single IC (Integrated Circuit) chip used for a dual interface card.
Recently, a non-contact battery-less IC card is increasingly used for an automatic ticket gate in transport facilities or an office entry. According to the non-contact type or technique, an IC card generates operating power from a radio wave contactlessly received from a reader/writer as a destination device.
A contact IC card is generally used for such systems as an ATM (Automated Teller Machine) for banks and an ETC (Electronic Toll Collection System) mounted on vehicles. According to the contact type or technique, an IC card is directly coupled to an ETC-equipped device as a destination device and is supplied with operating power from the ETC-equipped device while maintaining the coupled state.
There is a need from infrastructure users to carry as few cards as possible. Utilization information needs to be shared among infrastructures for sophisticated functionality. For these reasons, dual interface cards are increasingly used so as to be compatible with the contact and non-contact usages. In the dual interface card, the single IC chip provides both techniques. This is highly advantageous for embodying the dual interface card in terms of costs and capabilities.
However, the operating requirement of the IC chip greatly varies under the contact usage and the non-contact usage. In particular, the operating requirement of the non-contact usage needs to perform necessary processes while less power is consumed than the contact usage and the power consumption is prevented from being varied. Generally, the non-contact usage supplies the IC chip with a clock frequency lower than the contact usage.
In the IC chip in accordance with the conventional dual interface card, the electric energy supplied to the IC chip is monitored, and when the received electric energy is low, the function inside the IC chip is partially stopped, to reduce the power consumption. According to Patent Document 1, for example, an IC chip includes a CPU (Central Processing Unit), memory, and a coprocessor. When a result of monitoring the electric energy indicates the non-contact usage, the IC chip stops supplying clocks to the CPU and the memory during a coprocessor operation or to the coprocessor and the memory during a CPU operation. This reduces the power consumption. When a result of monitoring the electric energy indicates the contact usage, the IC chip allows the CPU, the memory, and the coprocessor to operate parallel. This ensures high-speed operations.
The IC chip uses the coprocessor that typically performs the encryption. Non-Patent Document 1 proposes a conventional technology that reduces power consumption of the coprocessor for encryption by miniaturizing the circuit scale and preventing variations in signal arrival times.    Patent Document 1: Japanese Unexamined Patent Publication No. 2004-206409, Non-contact IC card    Non-Patent Document 1: Morioka, S. and Satoh, A.: An Optimized S-Box Circuit Architecture for Low Power AES Design, CHES2002, LNCS Vol. 2523, pp. 172-186 (2002)